In general, a reluctance machine is an electric machine in which torque is produced by the tendency of a movable part to move into a position where the inductance of an energized phase winding is maximized. In one type of reluctance machine the energization of the phase windings occurs at a controlled frequency. These machines are generally referred to as synchronous reluctance machines. In another type of reluctance machine, circuitry is provided for detecting the position of the movable part (generally referred to as a "rotor") and energizing the phase windings as a function of the rotor's position. These types of machines are generally known as switched reluctance machines. The present invention is applicable to both synchronous and switched reluctance machines.
The general theory of the design and operation of reluctance machines in general, and switched reluctance machines in particular, is known in the art and is discussed, for example, in Stephenson and Blake, "The Characteristics, Design and Applications of Switched Reluctance Motors and Drives", Presented at the PCIM '93 Conference and Exhibition at Nuremberg, Germany, Jun. 21-24, 1993.
Most reluctance machines include a stationary member, called a "stator", that comprises a plurality of stacked laminations that include a number of projections (or "stator teeth") that define a plurality of discrete stator poles. Wire coils, typically copper, are placed within the gaps between the stator teeth to form one or more phase windings. The most common winding arrangement used to construct reluctance machines is the "single tooth" winding arrangement in which each stator tooth is surrounded by a single coil of wire. The various coils may then be linked together in series or parallel fashion to form one or more phase windings.
FIG. 1 provides a simple illustration of a traditional "single-tooth" reluctance machine comprising a six-pole stator 10 and a four-pole rotor 12. The laminations that comprise the stator have six inwardly projecting stator teeth 13-18 that define six stator poles. Each stator tooth 13-18 is surrounded by an individual coil of wire a.sub.1, a.sub.2, b.sub.1, b.sub.2, c.sub.1 and c.sub.2 and the coils surrounding stator teeth are electrically connected to form three phase windings A, B and C. The placement of the coils is reflected by the dots and crosses of FIG. 1 in which the crosses represent wire portions where positive electric current flows into the page and the dots represent wire portions where positive electric current flows out of the page. In typical operation of a single-tooth reluctance machine, each phase winding is energized for an interval corresponding to one-third of one complete rotor rotation such that each phase winding contributes to positive torque production one-third of the time.
Single-tooth reluctance machines, such as the one illustrated in FIG. 1, are somewhat limited in that the mechanism for torque production in such machines is exclusively a function of the self inductance of each energized phase winding. Because of the single-tooth nature of the windings, there is no mutual coupling between the machine phases and, thus, there is no significant torque production resulting from changes in the mutual inductances between the phase windings. As such, the maximum torque output and efficiency of reluctance machines with single-tooth windings is limited because the useful interval of such a winding is limited to those periods when the self inductance for that winding is increasing.
Despite the potential limitations of single-tooth winding configurations, the conventional wisdom of those working in the area of reluctance machines is that single-tooth windings are desirable because mutual inductance between the phase windings is, as a general matter, undesirable. The entrenched bias is expressed in publications concerning reluctance machines, including A. Hughes et al, "Effect of Operating Mode on Torque-Speed Characteristics of a VR Motor presented at the July 1976 International Conference on Stepping Motors and Systems" at Leeds University, Leeds, England, which contends that mutual inductance between phases diminishes the available torque production of three-phase reluctance machines with standard uni-polar excitations.
In contrast with the single-tooth machines discussed above--where there is no significant mutual inductance and all torque is produced as a function of self inductance--reluctance machine designs have been proposed where there is no self inductance and torque production is exclusively a function of changes in the mutual inductance between the phase windings. Such reluctance machines use fully-pitched winding arrangements. In general, a fully-pitched winding is a winding including winding coils that span M stator poles, where M is an integer equal to the number of phase windings. One such design was proposed by B. C. Mecrow in his paper entitled, "New Winding Configuration for Doubly Salient Reluctance Machines," published at the October 1992 IEEE Industry Applications Society Annual Meeting held in Houston, Tex.
FIG. 2 illustrates a reluctance machine using a fully-pitched winding of the type disclosed in the referenced Mecrow paper. In general, the machine includes a six-pole stator 20 and a four pole rotor 22 that are substantially identical in construction to the rotor 10 and stator 12 of the single-tooth machine of FIG. 1. The primary difference between the single-tooth machine of FIG. 1 and the fully-pitched machine of FIG. 2 is the placement and arrangement of the windings. In the fully-pitched machine of FIG. 3, there are only three winding coils a, b and c, and each coil is positioned within the stator such that the ends of the coils in the inter-pole gaps are offset from one another by 180 mechanical degrees for the illustrated six stator pole/four rotor pole design. Because of the fully-pitched nature of the windings of the machine of FIG. 2, virtually all of the torque production occurs as a result of changes in the mutual inductances between the two windings. As explained in the Mecrow paper, in such a machine, each phase winding is energized over a period covering two thirds of a complete rotor revolution such that each phase winding contributes, through changes in the mutual inductances between the two windings, to positive torque production for a period corresponding to two-thirds of each rotor rotation.
According to the referenced Mecrow paper, this fully-pitched, mutual-inductance only machine results in better utilization of the electromechanical circuit formed by the machine. Variants on Mecrow's fully-pitched reluctance machine are all discussed in P. G. Barrass, B. C. Mecrow & A. C. Clothier, "The Unipolar Operation of a Fully Pitched Winding Switched Reluctance Drives"; B. C. Mecrow, "Fully pitched-winding switched-reluctance and stepping-motor arrangements," IEE Proceedings-B, Vol. 40, No. 1 (January 1993); P. G. Barrass, B. C. Mecrow, & A. C. Clothier, "Bi-polar Operation of Fully-Pitched Winding Switched Reluctance Drives," International Conference on Machines and Drives (September 1995); and U.K. Patent GB 2,262,843 B, "Doubly salient reluctance machines."
The fully-pitched nature of the windings in a fully-pitched reluctance machine causes the various phase windings to have a fairly high self inductance when compared to the inductance of the phase windings in a single-tooth reluctance machine. This relatively high self inductance limits the rate of change of the current in the phase windings and thus restricts the speed at which the phase current can increase from zero to the peak value--and thus to the peak torque producing value. Accordingly, the fully-pitched nature of the windings in fully-pitched machine results in a "high self-inductance penalty" in that the drive used to power the motor must be sufficiently large to drive the currents to their desired value in an acceptable amount of time or the performance of the machine must be compromised because of the limitations placed on the phase current waveforms by the relatively high self inductance.
A further drawback of fully-pitched machines is that the amount of end-turn copper that is required to construct the fully-pitched coils used to form the phase windings. Such large and long end-turns result in increased manufacturing costs that can be directly tied to the amount of copper in the windings. Moreover, the relatively large amount of copper that is necessary to construct such fully-pitched windings results in increased resistance or copper losses during the normal operation of the machine.
It is an object of the present invention to provide an improved reluctance machine that overcomes the referenced and other limitations of single-tooth and fully-pitched reluctance machines.